1. Field of the Invention
The present invention relates generally to the mobile telecommunications field; and, more particularly, to an IP-based mobile telecommunications network that is capable of using compressed speech throughout the network.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a model of a GSM (Global System for Mobile Communications) telephony system. The GSM system model is generally designated by reference number 10 and includes a Radio Access Network (RAN) generally referred to as a Base Station System (BSS) 12. BSS 12 includes two types of logical nodes: a Base Transceiver Station (BTS) 14 and a Base Station Controller (BSC) 16. In order to support circuit-switched speech or data services, the BSC 16 interworks with a Mobile Switching Center (MSC) 18 via an open (non-proprietary) interface known as an A-interface (specified in GSM TS 08.08). An MSC, such as MSC 18, can serve one or more BSCs.
Each BSC in a GSM network can control a plurality (typically hundreds) of radio cells. In other words, each BSC, such as BSC 16, interworks with a plurality (hundreds) of (BTSs) via respective Abis interfaces. Each BTS, such as BTS 14, is responsible for the transmission and reception of radio signals over an air interface, Um, in one cell. Consequently, the number of cells in a GSM BSS equals the number of BTSs in that BSS. As such, the BTSs are geographically distributed to provide adequate radio coverage of a BSC area, which forms part of a GSM Public Land Mobile Network (PLMN).
Each BTS, such as BTS 14, provides the capacity to carry a plurality of connections (calls) between Mobile Stations (MSs), such as MS 22, and respective BSCs. Specifically, in GSM, each BTS is equipped with one or more Transceivers (TRXs). Each TRX (not shown) is capable of handling eight timeslots of a Time Division Multiple Access (TDMA) frame; and, in addition, each such timeslot can be assigned different combinations of logical channels.
FIG. 2 is a block diagram of an Internet Protocol (IP)-based BSS 30, which has been developed by Ericsson. A more detailed description of such an IP-based BSS is disclosed in commonly-assigned, co-pending U.S. application for patent Ser. No. 09/494,606, the entire disclosure of which is incorporated herein by reference.
Referring to FIG. 2, the IP-based BSS 30 can include three types of nodes connected to an IP network 32. A first node connected to the IP network 32 is a Radio Base Station (RBS) 34. In general, the RBS 34 implements one or more BTSs, transmits and receives calls from MSs 22 and provides IP support for the BSS 30. For example, the RBS 34 functions as an IP host and can include an IP router (not shown in FIG. 2). The IP router can be used to route payload User Datagram Protocol (UDP) datagrams to one or more Transceivers (TRXs) and also to connect a plurality of RBSs in various topologies.
A second node connected to the IP network 32 is a GateWay (GW) 36. The GW 36 can be used to terminate the A-interface, and can include a Media GW (MGW), not shown in FIG. 2 but which will be described more fully hereinafter, which functions similarly to existing Transcoder Controllers in an Ericsson implementation of the GSM model. The MGW includes a pool of Transcoder/Rate Adaptor (TRA) devices, which, when allocated, are connected to the A-interface; and, hence to the MSC 18 via the A-interface. The IP network (e.g., GSM) side of the TRAs in the MGW are connected to respective UDP ports. Preferably, the GW 36 is connected to the IP network 32 via a separate router (not shown).
A third node connected to the IP network 32 is a Radio Network Server (RN Server) 38. The RN Server 38 corresponds to the BSC used for implementing a GSM model, such as the GSM model 10 illustrated in FIG. 1. A primary difference between the RN Server 38 and a BSC is that the RN Server does not switch payloads and does not include a Group Switch (GS). As such, the RN Server 38 preferably carries signaling only, and includes a pool of processors (e.g., the number of processors determined by capacity requirements). The RN Server 38 can serve one or more logical BSCs and is preferably connected to the IP network 32 via a separate router. As such, the payload can be routed directly between the GW 36 and RBS 34, without passing through the RN Server's processors. The A-interface signaling is routed between the RN Server 38 and GW 36.
FIG. 3 schematically illustrates an exemplary mobile telecommunications network operating in accordance with GSM specifications. The network is generally designated by reference number 50, and comprises three geographical areas 52, 54 and 56, also designated in the Fig. as areas G, S and M, respectively. Each geographical area 52, 54 and 56 includes a connection 58 to a PSTN (Public Switched Telephone Network), and each area also includes a plurality of RBSs 62 to provide full radio coverage. Each RBS is connected to a BSC 64 where transcoders (not shown in the Fig.) are located. The BSCs, in turn, are connected to MSCs 66 in each area.
In the network 50 illustrated in FIG. 2, voice traffic is carried from an MS (not shown in FIG. 2) to the transcoders in a BSC 64 via an RBS 62 on 8 or 16 kbits/sec channels. From the BSC, the voice traffic is carried on 64 kbits/sec channels to the MSC 66 (via the A-interface), and further through the MSC 66 to the PSTN connection 58.
Thus, in a GSM system, compressed speech can be used only between the MS and the BSC. From the BSC to the MSC and from the MSC to the PSTN connection, voice traffic must be carried on 64 kbits/sec channels. This results in high transmission costs for the system operator.
In order to assist in understanding the present invention, an example of the operation of a GSM network such as illustrated in FIG. 3 will now be described. In the example, a scenario with an MS terminating call will be described; and in such a scenario, as described above, it is only possible to use compressed speech from the BSC to the MS. The MS terminating call is made in the following way.                A PSTN-sub in area S calls an MS-sub in area M.        The PSTN connects the call to the nearest gateway MSC (MSCS).        The MSCS forwards the call to MSCM.        MSCM sends a page to BSCM.        BSCM sends the page to all RBSs in area M.        When the MS answers, BSCM sets up a signaling connection to MSCM.        MSCM selects a CIC (Circuit Identity Code) in MSCM and sends the CIC value to BSCM in the Assignment Request.        When BSCM receives the Assignment Request, BSCM selects a TRA in BSCM and connects it to the RBS in area M.CIC is always selected in the MSC closest to the BSC resulting in a 64 kbits/sec connection from the PSTN in area S to the BSC in area M, and compressed speech (e.g., 8 or 16 kbits/sec) from the BSC in area M to the MS.        
In recent years, substantial effort has gone into development of so-called 3rd generation mobile telecommunications systems in order to address the growing demand for wireless multimedia services. One implementation of a 3rd generation system is known as the Universal Mobile Telephony System (UMTS); and FIG. 4 schematically illustrates a UMTS system.
The UMTS system is generally designated by reference number 70, and is configured in accordance with the 3rd Generation Partnership Project (3GPP) technical specifications. UMTS 70 includes a Core Network 72, and a Universal Terrestrial Radio Access Network (UTRAN) 74. UTRAN 74 includes one or more Radio Network Subsystems (RNSs), such as RNSs 76a and 76b. The RNSs 76a and 76b each include an RNC (Radio Network Controller) 78a and 78b, respectively, and related Node Bs 80a, 80b and 80c, 80d, respectively.
The Core Network 72 enables subscribers to access services from a network operator. An RNS can function in a UTRAN as the access part of the UMTS network; and can allocate and release specific radio resources in order to establish connections between a UTRAN and a mobile station 82 as shown in FIG. 4. Thus, an RNS is generally responsible for the radio resources and transmission/reception in a set of cells. The RNCs in the RNSs generally function to control the use and integrity of radio resources. Each Node B is a logical node responsible for the radio transmission/reception in one or more cells and to or from an MS. A Node B is generally similar to a base station in a non-3rd generation system. An RNC, e.g., RNC 78b, can function as a Controlling RNC (CRNC) with respect to a specific set of Node Bs. A Node B, however, typically has only one CRNC. A CRNC generally controls the logical resources of its related Node Bs. As shown in FIG. 4, an RNC and a Node B communicate with one another via an Iub interface, RNCs communicate with one another via an Iur interface and RNCs communicate with the Core Network via an Iu interface.
One of the drivers for the function distribution for the Iu interface in a UMTS system was to make it possible to have the transcoder at the edge of a PLMN. As indicated above, however, this has not been possible with the A-interface in a GSM system.